Annular mesh

ABSTRACT

An annular mesh expandable radially from a compact diameter to a radially-expanded deployed disposition in which the mesh is capable of sustaining a radially outwardly directed resistive force even when flexing its longitudinal axis out of a straight line, the mesh being composed of stenting struts, the stenting struts being arranged in a plurality of zig-zag strings around the circumference of the lumen, with occasional connector struts joining adjacent strings to create a closed circumference unit cell between two such connector struts and two adjacent connected strings there being a plurality of such unit cells arranged in sequence around the circumference between said two adjacent strings; and characterised in that there is a non-constant increment of strut length, serving to displace along the longitudinal axis each unit cell relative to the circumferentially next adjacent unit cell.

FIELD OF THE INVENTION

This invention relates to an annular mesh that defines a lumen thatsurrounds a longitudinal axis of the mesh, the mesh being capable ofsustaining a radially outwardly directed resistive force even whileflexing in response to externally applied forces that bend itslongitudinal axis out of a straight line, the mesh being composed ofstruts, the struts defining a plurality of repeating unit cells eachwith a closed periphery, a string of said unit cells providing each of aplurality of stenting loops that surround said lumen.

BACKGROUND ART

Such an annular mesh is the operative part of a bodily prosthesis thatis commonly known as a stent. The purpose of the stent is to maintain abodily lumen patent and, to do this, the mesh of the stent must resistthe radially inward pressure of the bodily tissue that would otherwiseclose the bodily lumen.

As usage of annular mesh stents becomes ever more sophisticated, thedemands for the annular mesh to be flexible, even while it resistsradially inward pressure from bodily tissue, also increase. Stentdesigners have found it difficult to increase flexibility (in responseto requirements for the longitudinal axis of the mesh to bend out of astraight line) while retaining adequate resistance to radially inwardforces on the mesh.

Readers will readily appreciate that improvements in stent design couldyield annular meshes that are interesting for application beyond bodilyprostheses, whenever a combination of good resistance to radially inwardforce, and good bending flexibility, is required. The present inventionmay have applications beyond bodily prostheses and therefore thedefinition of the present invention refrains from limitation to stents.

Up to now, there have been two archetypal stent mesh designs, the firstexhibiting a sequence of stenting rings each of which is a closed looparound the longitudinal axis. Adjacent stenting rings need to beconnected so as to maintain a predetermined spacing between adjacentstenting rings along the length of the stent. Individual stenting ringshave little or no capacity to bend when the longitudinal axis of theannular mesh is urged by external forces into a bent rather than astraight line, so the connectors between adjacent stenting rings carrymost of the strain that allows such bending. Increasing the number ofconnectors increases the rigidity of the mesh, but an insufficientnumber of connectors can prejudice the integrity of the mesh. Inconsequence, many of the connectors evident in commercial stents arelong and serpentine rather than short and straight. For examples of ringstents, see for example U.S. Pat. No. 6,770,089, US2002/0116051 and WOpublications 2005/067816, 96/26689, 99/55253 and 03/055414.

The other characteristic form of a stent mesh is the helical stent, inwhich stenting struts proceed as zig zags around a spiral path from oneend of the stent to the other. Connectors may be provided at spacedintervals, between successive turns of the spiral, for locationalintegrity of the mesh. A spiral form mesh has inherently moreflexibility, and less resistance to radially inwardly directed forces,than is the case with a stack of closed stenting loops arrangedtransverse to the longitudinal axis of the annulus of the stent. Forexamples of helical stents, see for example, EP-A-1245203 and 870483,U.S. Pat. No. 6,053,940 and WO publications 2002/049544 and 01/018839.

Considering both the “ring stent” and “helical stent” categories,stenting loops advance around the circumference of the stent lumen as azig-zag of stent struts that alternate with zones of inflection. Takingthe line that is the bisector of the angle between two adjacent strutsof a zig-zag loop, that bisector will likely lie parallel or nearparallel to the longitudinal axis of the stent lumen, in any ring stent.Conversely, in any helical stent, that bisector will likely lie at anangle to the longitudinal axis, that is larger as the helical pitch ofthe stenting loops gets larger.

SUMMARY OF THE INVENTION

The present inventor recognized the advantages in retaining something ofthe flexibility of the spiral stent mesh, together with something of theradial force capability of closed stenting loops. This he accomplishesby providing a unit cell for the stent matrix, which resembles that of aring stent yet, when assembled into the stent matrix, yields a spiralwind of unit cells around the longitudinal axis of the stent.

The point can be illustrated by a chessboard. A ring stent is like arook (castle). The zig-zag stenting ring advances around thecircumference strictly perpendicular to the longitudinal axis of thestent lumen. A helical stent is like the bishop. He advances in astraight line again, but slanting to the long axis of the lumen.Embodiments of the present invention exhibit a path of advance of thezig-zag stenting loops like the way the knight moves—forwards, thenacross, then forwards again.

In one embodiment of annular mesh in accordance with the presentinvention, successive stenting loops are joined end-to-end in acontinuous spiral around the longitudinal axis of the annulus, but thesuccessive turns of the spiral are made out of a plurality of unitcells, each with a closed periphery. The closed cells contribute agreater capability than a pure spiral stent to resist externally imposedradially inwardly directed forces, while the spiral architecture of themesh contributes more flexibility than a closed loop stent mesh. Not allstruts that make up the periphery of the unit cell share the samelength. By introducing a strut length increment within the periphery,the invention can be realised.

Thinking about conventional ring stents, typically all struts in a unitcell have the same length. As for conventional spiral stents, the strutstypically exhibit a constant incremental shift of the position of thestruts along the longitudinal axis of the stent. In the presentinvention, the periphery of the unit cell is characterised by adeparture form this degree of uniformity.

Typically, in an annular mesh in accordance with the present invention,there is a strut that is shared by two adjacent unit cells, that strutcontributing to the periphery of both of the two adjacent cells. In oneembodiment, each such strut has a length direction that is parallel tothe longitudinal axis. In other embodiments, the length direction of thestrut is not parallel to the longitudinal axis.

Looking at the unit cells of prior art closed loop stenting meshes, itis often possible to identify a unit cell that exhibits mirror symmetryabout a plane that includes the longitudinal axis of the mesh.Typically, in the present invention, the unit cell lacks such mirrorsymmetry. Instead, the unit cell of the present invention typicallyexhibits 180° rotational symmetry about a rotational axis that isperpendicular to the longitudinal axis of the annular mesh and alsointersects the longitudinal axis of the annulus. In stent strutmatrices, simplicity is a desirable objective. If only for this reason,stent unit cells with 180° rotational symmetry are preferred.

Below is a detailed description of individual embodiments, which helpsto make this more clear.

The mesh of the present invention finds particular application in astent for transluminal implantation in a body, and that stent may be,for example, a self-expanding stent or a balloon expansible stent.Conveniently, the annular mesh is formed from a sheet-form workpieceand, although that workpiece could be a flat sheet, it is desirably inthe form of a seamless tube. The art of creating a stent mesh by cuttingslits in a workpiece is by now quite well known. Typically, acomputer-controlled laser is employed to cut slits in a seamless tubularworkpiece held on a jig and, typically, the slits are parallel to thelong axis of the workpiece.

While an annular mesh in accordance with the present invention typicallydisplays a single spiral of the unit cells, that exhibits a pitch thatcorresponds to the length of the unit cell in the longitudinal axisdirection of the annulus, this need not be so. One envisages meshes thatexhibit a double spiral, but this is unlikely to be preferred, becauseof the constraints which such a design imposes on the dimensions of theunit cell.

Desirably, the spiral pattern mesh will exhibit connectors regularlyarranged around the circumference of the annulus with the view tomaintaining desired axial spacing between successive adjacent turns ofthe spiral. Preferably, these connectors are staggered circumferentiallywith respect to the connectors joining the next adjacent pair ofstenting loops around the annulus, for optimising the balance betweenflexibility and structural integrity. With the inherent flexibility of aspiral pattern, one envisages the possibility of using simple straightstruts as connectors, and not needing to resort to the lengthy orserpentine connectors often found in prior art stent meshes. Simplicityof design is always an advantage, and stent architecture is noexception.

One way of recognising the hybrid nature of the stents according to thepresent invention is to contemplate the orientation of the bisector ofthe angle between the struts that form any one zig-zag of the mesh. Itwill tend to be parallel or near parallel to the longitudinal axis ofthe stent lumen, more like a ring stent than a helical stent. Yet,overall, the mesh of the present stents is recognisably a helicalpattern rather than a stack of closed stenting loops.

The inherent flexibility and radial strength of the annular mesh of thepresent invention will be useful not only for bare stents but also forthe stent meshes used in grafts, or other annular meshes used insurgical tools such as vascular filters, in which the mesh would be usedto carry a filter membrane.

For a better understanding of the present invention and to show moreclearly how the same may be carried into effect, reference will now bemade, by way of example, to the accompanying drawings. These areincorporated herein and constitute part of this specification. Theyillustrate presently preferred embodiments of the invention and,together with the general description above, and the detaileddescription below, serve to explain the features of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1 to 4 each show a view of an annular mesh, opened out flat in aradially expanded configuration;

FIG. 5 shows a fragment of a laser cutting pattern on a tubularworkpiece laid out flat;

FIG. 6 shows at smaller scale the complete workpiece of FIG. 5;

FIG. 7 is a photograph of part of the FIG. 6 workpiece after radialexpansion;

FIG. 8 is a schematic representation of the strut pattern of FIG. 7; and

FIG. 9 is a diagram of the way three chess pieces more over achessboard.

DETAILED DESCRIPTION

It is convenient, for two dimensional drawing sheets, representing anannular mesh matrix, to open the matrix out and lay it flat on the planeof the drawing sheet. This has been done, in each of the accompanyingdrawing FIGS. 1 to 4, 5, 6 and 8. The skilled reader will appreciatethat each of drawing FIGS. 1 to 4, 5, 7 and 8 shows only arepresentative portion of the annular mesh, enough to reveal thecharacteristic repeating unit cell so that the reader can complete therest of the annular mesh. In each case, the longitudinal axis of theannular mesh extends horizontally across the drawing page. Thus, forinvestigation of mirror symmetry of one of the unit cells displayed inany of the attached drawing sheet, one considers symmetry across a planethat extends perpendicular to the plane of the page, and lies in theEast-West direction. For rotational symmetry, the relevant rotationalaxis is one that extends perpendicularly upwards out of the plane of thedrawing page. After having considered the drawings, and theidentification of the respective unit cell, the reader will be able todetermine and confirm that none of the illustrated unit cells displayssymmetry across a mirror plane as thus defined but all of the unit cellsdisplay 180° rotational symmetry about a rotational axis extendingperpendicularly upwards from the plane of the drawing.

Looking first at drawing FIG. 1, we can ascertain in the drawing aportion of the mesh that includes four slanting zig-zag strings ofstenting struts, marked A, B, C and D. The repeating unit cell is markedU1 and there are two such unit cells needed to make up one complete turnaround the circumference of the annulus. The zig-zag strings A and B arebridged by only two connector struts 10 and 12, it being appreciatedthat strut 12 between strings A and B at the foot of the page is thesame as strut 16 between strings C and D at the top of the page ofdrawing FIG. 1 connecting zig-zag strings C and D. Likewise, zig-zagstrings C and D are connected by only two struts 14 and 16.

The connector struts 13 and 20 that connect zig-zag strings B and C arecircumferentially staggered relative to the struts 10, 12, 14 and 16that make up parts of the periphery of the unit cells in the nextadjacent stenting loops, formed by strings A and B and strings C and Drespectively. There are two unit cells U1 lying between zig-zag string Band zig-zag string C. One of those unit cells is evident, in full, inFIG. 1. The other is just as much a closed periphery cell, even thoughin FIG. 1 it appears in two separate halves, both with an incompleteperiphery, and marked U2 t at the top of the page and U2 b at the bottomof the page. In other words U2 t+U2 b=U1.

The drawing shows the annulus in a radially expanded disposition, readyto resist radially inwardly directed forces tending to reduce thediameter of the lumen surrounded by the annular mesh. Supposing that themesh is representative of the strut matrix of a transluminally deliveredstent, one can imagine the disposition of the struts shown in FIG. 1when the strut is in a radially small transluminal delivery disposition,by compressing the strut arrangement presented in FIG. 1, in aNorth-South direction on the drawing page (supposing North is at the topof the page). The skilled reader will appreciate that all of the strutsconcertina or fold down to a disposition in which they are lying more orless horizontally, East-West, on the drawing page. One can furtherimagine how such a strut matrix can be made from a seamless tubularworkpiece, by laser cutting slits in the workpiece, to leave strutshaving the wall thickness of the annular workpiece, all such slits, andall such struts, having a length direction parallel to the East-Westlongitudinal axis of the annular mesh shown in the drawing.

It will be evident that the design shown in FIG. 1 is of unit cells withshared boundaries, obviating the need for any connector struts betweenadjacent unit cells that do not themselves constitute a part of theclosed periphery of one or other unit cell of the matrix. One canimagine struts 18 an 20 as connectors, connecting like unit cells lyingbetween zig-zag strings A and B with end cells located between zig-zagstrings A and B. However, struts 18 and 20 are themselves part of theclosed periphery of unit cells of the same form, lying between zig-zagstrings B and C.

Also evident from FIG. 1 is that not all the stenting struts in eachzig-zag string A, B, C, D have the same length. Struts such as strut 22are notably short. In each unit cell U1, we can discern two of the shortstruts 22. Rotating the unit cell by 180° about a rotational axisperpendicular to the plane of the page will bring each of the two shortstruts to the location of the other of the two short struts in any ofthe unit cells.

Turning to FIG. 2, presented is a unit cell that exhibits around itsperiphery a larger number of struts than is evident from the unit cellof FIG. 1. In FIG. 2, a string of only three unit cells is needed tocomplete a circumference of the annulus of the mesh. It will again beappreciated that unit cell U3 has 180° rotational symmetry but notmirror plane symmetry. Imagining the mesh compressed in the North-Southdirection on the page, one can appreciate that, in the radiallycompressed disposition of the mesh, all slits and all struts extendEast-West on the page, parallel to the longitudinal axis of the annulus.

The design of FIG. 3 differs from the designs of FIG. 1 and FIG. 2 inthat the adjacent loops of unit cells U4 are spaced from each otherlongitudinally (East-West in the drawing) but joined to each otherlongitudinally by connector struts 40, of which there are four, spacedby equal intervals around the circumference of the annulus. Compared tothe unit cell U1 of FIG. 1, each unit cell U4 has around its peripheryfour more stenting struts, two in each zig-zag string. This extrazig-zag in the circumferential length (North-South in the drawing) ofeach unit cell U4 provides the scope for placing connector struts 40 atlocations intermediate the circumferential end struts 42, 44 of each ofthe unit cells. Interesting is that the cells of each stenting loop offour unit cells U4 are not circumferentially staggered relative to thecells of the axially next adjacent string of unit cells U4. However,lying axially between the two shown strings of unit cells U4 is a stringof 4 unit cells U5 that are alike with each other but different from theunit cell U4.

The view from any unit cell U4, looking along the longitudinal axis ofthe annulus, is of other unit cells U4, located (in the drawing) dueEast and due West of the viewing position, without any circumferentialstagger towards the North or the South. Nevertheless, by virtue of thedifferent length struts, any particular peak point of inflection 46faces in the longitudinal direction of the annulus a valley 48 betweentwo struts 50 and 52 of the next adjacent unit cell linked by theconnector strut 40. Thus, the arrangement of unit cells and connectorstruts is relatively simple (all struts are cut straight) yet the“peak-to-valley” configuration in the radially expanded dispositionshown in FIG. 3 has the merit that flexing of the annular mesh, when thelongitudinal axis is bent out of a straight line, does not have thetendency to bring peaks like peak 46 into face-to-face contact withequivalent peaks on a next adjacent stenting loop but, rather tends tobring them down into a valley such as valley 48. Such a configuration isparticularly attractive to have, on the inside of the bend, when theannular mesh is being deformed around a tight radius.

It will also be appreciated by skilled readers that although struts 40are on a line that slants relative to the longitudinal axis of theannulus, when the mesh is in the expanded disposition as shown in FIG.3, that same strut 40, in the radially compact disposition of the meshwill be parallel to the longitudinal axis. Again, the mesh of FIG. 3 isone that can be made from a seamless tubular workpiece by cutting slitsall parallel to the long axis of the tube, to create struts all parallelto the long axis of the tube, which only depart from such a parallel(East-West in FIG. 3) direction, when the mesh is radially expanded upinto the opened out zig-zag configuration to be seen from FIG. 3. Again,this is a win for simplicity, this time in the slit- and strut-cuttingpart of the manufacturing process.

Finally, the unit cell of FIG. 4, U6 is not unlike the one presented inFIG. 2. The most evident difference is the orientation of the strut 50that is shared between two adjacent unit cells of the stenting loop.Although the connector strut 50 is slanting to the long axis of theannulus when viewed in the expanded configuration of FIG. 4, once again,concertina compression North-South of the mesh of FIG. 4 down to theradially compact disposition in which an annular workpiece can beslitted to provide the strut seen in FIG. 4 will bring strut 50 into anorientation in which it is parallel to the East-West longitudinal axisof the annulus of the FIG. 4 mesh.

It will be appreciated that the unit cells of FIGS. 2 and 4 share withthose of FIG. 1 the characteristic that the peripheral stenting strutsof each unit cell (except connector strut 50), are shared with theadjacent unit cell lying respectively East and West of the unit cellseen in FIG. 4, that is, next adjacent in the longitudinal direction ofthe annulus of the mesh. Struts 50 are connectors, but also strutscontributing to the closed periphery of a unit cell U5. There are noconnectors in the annular meshes of any of FIGS. 1, 2 and 4 which arenot also struts that are part of the periphery and closed circumferenceof a unit cell of the matrix.

Although the various unit cells of FIGS. 1-4 have been describedindividually, it is intended that various combinations of the respectiveunit cells of FIGS. 1-4 be utilized for a stent framework. For example,a stent framework can be constructed, in sequence, unit cells of FIG. 1connected to the unit cells of FIG. 2, which are connected to the unitcells of FIG. 3, which are also connected to the unit cells of FIG. 4,i.e., FIG. 1-FIG. 4-FIG. 2-FIG. 3, FIG. 4-FIG. 1-FIG. 2-FIG. 3 and so onin at least 24 different permutations.

Attention is now directed to drawing FIGS. 5 to 8. FIG. 6 shows a stentmatrix from one end to the other, laid out flat. FIG. 5 shows a middleportion of the length of the FIG. 6 stent matrix. Thus, features of therepeating stenting strut matrix will be described by reference to FIG.5, where the dimensions are bigger, whereas features visible only at theends of the stent matrix will be described by reference to FIG. 6.

Looking at FIG. 5, we see a multitude of slits that are linear andparallel to the longitudinal axis of the stent lumen, each stentingstrut having a constant width in the circumferential direction of thematrix. For clarity, only one of the struts 110 has a lead line andreference number, only one of the laser cut slits 112 and in only onelocation, reference W, the characteristic circumferential width of thestenting strut as indicated.

Overall, the matrix displays a slanting or helical pattern, in that eachsequence of stenting struts alternating with points of inflection 114,is seen to lie between notional slanting lines S1 and S2 that lie at anacute angle to the longitudinal axis of the matrix. By contrast, aso-called “ring stent” would display stenting rings between two notionallines that are perpendicular to the longitudinal axis of the stent.

Each of the inclined zig-zag stenting loops is joined to the adjacentstenting loop by connector struts 116 that are seen to have acircumferential width of 2W and that extend across the slanting lines S1and S2 with a length direction parallel to the long axis of the stent.As can be see, there are four such connector struts 116 in each turn ofthe stenting loop around the axis of the matrix. It is part of theadvantage of the invention that it can yield stents with a highflexibility even through stenting loops are connected by a plurality ofsimple short straight axial connectors.

It can also be seen that there are “holes” 113 in the matrix, that is,through apertures in the stent wall, of substantial open area, even inthe as cut matrix, which holes also span the slant line S1, S2 and liebetween two adjacent stenting loops. These holes 118 arise during thelaser cutting of the stent matrix, after the laser has cut all aroundthe periphery of hole 118. See our own earlier WO 2001/032102 for adescription of the creation of similar holes.

Importantly, each stenting loop 120 that lies between two slantinglines, for example S1 and S2, exhibits loop portions with a constantlength of laser cut 112 and stenting strut 110. Furthermore, these cuts,struts and points of inflection 114 present an appearance of part of aring stent with its stenting loops perpendicular to the long axis of thestent. They are interspersed by occasional, shorter than usual struts122 that are contiguous with a connector strut 116. The skilled readerwill appreciate that the response of a strut to any particular appliedbending stress depends on the length of that strut.

Before leaving FIG. 5 it is important to note that every point ofinflection 114 is facing “head-on” a point of inflection of the nextadjacent stenting loop, either across a gap 118 or indeed “nose-to-nose”as at 124.

Moving to FIG. 6, we see in particular an architecture at each end ofthe stent, between the slanting architecture of the main length of thestent and the end stenting rings 130, one at each end of the stent.Connecting the slanting architecture to the perpendicular end ring arefour connector struts distributed evenly around the circumference of thelumen, but these four struts, 132, 134, 136 and 138 are of differentlengths and widths. The reader will appreciate that the meeting of aspiral pattern and a perpendicular end ring is necessarily going to giverise to “holes” between the slanting and perpendicular architecture thatare unsymmetrical. The holes in the present case are referenced 140,142, 144 and 146. Hole 140, in particular, is relatively large in theaxial direction.

Turning now to drawing FIGS. 7 and 8, we see the FIGS. 5 and 6 matrix inexpanded disposition, as it would be when working to hold radially opena bodily lumen. FIG. 7 is useful because it is a photomicrograph of areal embodiment, so that it gives an impression of the way in whichshorter struts bend less than longer ones. FIG. 8 is schematic and doesnot show this reality. Its suggestion that every strut is inclined atthe same constant angle to the longitudinal axis of the stent lumen, inthe expanded configuration, is not the reality. Neither does FIG. 8preserve the relative scale of the length difference between the regularstrut length 110 and the occasional “special” strut length 122. Inreality, as seen in FIG. 7 and FIGS. 5 and 6, the occasional short strutis nearly as long as the normal strut 110. However, FIG. 8 is useful inshowing how points of inflection 114 no longer face each other head on,across a short axial gap. At those locations where the points ofinflection would be “nose-to-nose” especially on the inside of a bendwhen the stent is bent into a banana shape, as might happen in normaluse in peripheral vascular applications, we see the points of inflectionto be circumferentially staggered relative to each other so that thereis a minimal risk of them impacting nose-to-nose, on the inside of thebend, in use of the stent.

Notably, the presence of distinct “holes” 118, in FIGS. 5 and 6 is nolonger apparent in the stenting disposition of FIGS. 7 and 8. Instead,these zig-zag strut patterns give every appearance of providing uniformcover for the entirety of the bodily lumen to be stented, without anylonger any “holes” through which bodily tissue can easily pass.

It should also be noted that the huge increase in circumferentiallength, on moving from the as cut configuration of FIG. 5 to the workingdisposition of FIGS. 7 and 8 has the effect of bringing the slant angle(S1, S2) back much closer to perpendicular to the long axis of thelumen. Whereas the cut pattern of FIG. 5 looks strikingly helical, thestrut pattern being displayed in FIGS. 7 and 8 looks much closer to thatof a ring stent with axially spaced, discrete, endless stenting loops.It will appreciated that the greatest radial force that can be generatedby a particular stent architecture is when the zig-zag loops arearranged in endless loops, discrete, and in a stack spaced along theaxis of the stent. FIG. 7 is quite a close approximation to this ideal.

FIG. 8 serves another useful purpose, to render more easily visible thehybrid (intermediate between “ring” and “spiral” stent) nature of theembodiment.

We see in the strut matrix a plurality of nodes 160, where three strutsend, one of which is a connector strut 116. In this embodiment each node160 is also a point of inflection 114 between a regular stenting strut110 and one of the occasional shorter struts 122.

Let us examine the zig-zag string of stenting struts that includes thethree leg nodes 16A, B, C and D. Each portion of the zig-zag string,between any two adjacent nodes A-B, B-C, C-D, is a portion of a stentingring that is orientated perpendicular to the stent axis. Eachcircumferential portion A to B, B to C, C to D is axially stepwiseoffset from its neighbour portions, the step occurring at the three legnode. The same can be discerned in the other drawing figures but theschematic representation of FIG. 8 makes it a little easier torecognise. Making, in FIG. 8, the short struts 122 exaggeratedly shorteralso helps to reveal the effect.

Looking at FIG. 8 and concentrating on a unit cell with a circumferencethat displays 6 three leg nodes, one can recognise two pairs of nodesdefining the corners of the unit cell at each circumferential end of thecell. In unit cell 170, for example, nodes 160B and 160D are each amember of such a pair. Part way along each of the two long sides of thecell is a further three leg node 160C and 160E. In the middle portion of170H of the length of the unit cell 170, the left hand (in the drawing)wall of the cell above node 160E is stepped axially to the left at node160E, but the right hand wall does not do so until node 160C. Thus, theportion 170H corresponds to a “hole” 118. But it does not look much likea “hole” in the “real life” photograph of FIG. 7 because thedifferential bending performance of the different length struts arecompensating for the effect, and thereby tending to “fill” the hole withstent strut zig-zag portions. This is an incidence of serendipity, ahappy unanticipated coincidence which enhances the performance of thedesign.

The stent strut matrix designs of the present invention lend themselvesto covered stent embodiments, such as stent grafts. The “holes” 118 canoffer good possibilities for bonding together, across the stent wall,films or membranes (such as of expanded PTFE) that lie radially withinand outside the stent annulus. The state of the art is replete withteachings how to apply coatings to stents but a stent matrix inaccordance with this invention offers possibilities not available withprior art stent strut matrices.

The layout of struts in the real life expanded configuration (such asshown in FIG. 7) can be managed by somehow restraining the struts on ajig, in the layout desired, during heat-setting of the stent shape whichis to be “remembered” by its shape memory alloy. This can be done, forexample, by heat-setting the stent matrix on a mandrel which has beenengraved with the desired stent pattern. The struts are accommodated inthe troughs of the engraving, during the heat-setting step.

Finally we turn to FIG. 9 of the drawings, and the reference earlier inthis description to the way in which pieces move on a chessboard.Looking at the diagram of FIG. 9, length along the stent lumen isindicated by the X-axis L, whereas movement around the circumference ofthe stent lumen is indicated by progress along the Y-axis marked C. Fora ring stent we use the symbol for the chess piece called the “rook” or“castle” and this of course is a line that is perpendicular to the longaxis of the stent and that extends simply all the way around thecircumference. Likewise, a helical stent, like a spiral wire that“unwinds” when placed in a bodily lumen (the original “Dotter”prosthesis) can be represented by the symbol for a “bishop” chess piece,that moves sideways as much as it moves up and down the chessboard, butalways in a straight line.

By contrast, the “knight” chess piece follows a distinctive path thatcan be characterised as “two steps forward and one step across” (theknight has other possibilities such as “two steps across followed by onestep backwards” but for the purposes of the present description we neednot concern ourselves with these other possibilities. Important tostress is that the present invention seeks to take the best of both thering stent and the spiral stent, and to do this by building stentingloops that, in one sense, advance around the circumference in adirection perpendicular to the stent axis while, in another sense,spiraling around the lumen. To do this, the zigzag loops can advance fora circumferential portion perpendicular to the axis and then stepaxially sideways, before resuming their advance, for another smallportion of the circumference, perpendicular to the longitudinal axis ofthe annulus. See FIGS. 5 and 6.

The aim to achieve the “best of both worlds” is of course to achieve theradial force of a ring stent with the flexibility of a spiral stent.Intuitively, one can see from FIG. 7 that the embodiments of the presentinvention probably do provide a high proportion of the stenting force ofa ring stent. When it comes to flexibility, one can also intuitively seefrom FIGS. 7 and 8 that bending the stent into a banana shape isprobably going to be achievable without too much local overstressing ofthe material of the stent, even to the extent of allowing points ofinflection of the zig-zag stenting rings to pass by each other as theyenter the substantially triangular spaces available between two strutsof the adjacent stenting ring, the impact of points of inflection,nose-to-nose being generally avoided.

In this way, stents according to the present invention offer thepossibility of achieving, simultaneously, both a high radial stentingforce and a high tolerance of bending after placement in the body.

Where undulations are embodied in the form of zig-zag struts, thezig-zag struts may include a repeating pattern made of an unit of fourgenerally linear members that extend oblique to the longitudinal axis tointersect each other at three apices spaced apart circumferentially andaxially. Also, the prosthesis can utilize not only the circumferentialbridges but also other bridge configurations in combination.Alternatively, the bridge directly connects a peak of onecircumferential section to another peak of an adjacent circumferentialsection. In yet another alternative, the bridge may connect a peak ofone circumferential section to a trough of an adjacent circumferentialsection. In a further alternative, the bridge can connect a trough ofone circumferential section to a trough of an adjacent circumferentialsection. Moreover, the undulations can be wave-like in pattern. Thewave-like pattern can also be generally sinusoidal in that the patternmay have the general form of a sine wave, whether or not such wave canbe defined by a mathematical function. Alternatively, any wave-likeforms can be employed so long as it has amplitude and displacement. Forexample, a square wave, saw tooth wave, or any applicable wave-likepattern defined by the struts where the struts have substantially equallengths or unequal lengths. And as used herein, the term “implantableprosthesis” is intended to cover not only a bare stent but also coated,covered, encapsulated, bio-resorbable stent or any portion of similarstents.

Bio-active agents can be added to the prosthesis (e.g., either by acoating or via a carrier medium such as resorbable polymers) fordelivery to the holt's vessel or duct. The bio-active agents may also beused to coat the entire stent. A material forming the stent or coupledto the stent may include one or more (a) non-genetic therapeutic agents,(b) genetic materials, (c) cells and combinations thereof with (d) otherpolymeric materials.

-   -   (a) Non-genetic therapeutic agents include anti-thrombogenic        agents such as heparin, heparin derivatives, urokinase, and        PPack (dextrophenylalanine proline arginine chloromethylketone);        anti-proliferative agents such as enoxaprin, angiopeptin, or        monoclonal antibodies capable of blocking smooth muscle cell        proliferation, hirudin, and acetylsalicylic acid;        anti-inflammatory agents such as dexamethasone, prednisolone,        corticosterone, budesonide, estrogen, sulfasalazine, and        mesalamine; antineoplastic/antiproliferative/anti-miotic agents        such as paclitaxel, 5-fluorouracil, cisplatin, vinblastine,        vincristine, epothilones, endostatin, angiostatin and thymidine        kinase inhibitors; anesthetic agents such as lidocaine,        bupivacaine, and ropivacaine; anti-coagulants, an RGD        peptide-containing compound, heparin, antithrombin compounds,        platelet receptor antagonists, anti-thrombin antibodies,        anti-platelet receptor antibodies, aspirin, prostaglandin        inhibitors, platelet inhibitors and tick antiplatelet peptides;        vascular cell growth promoters such as growth factor inhibitors,        growth factor receptor antagonists, transcriptional activators,        and translational promoters; vascular cell growth inhibitors        such as growth factor inhibitors, growth factor receptor        antagonists, transcriptional repressors, translational        repressors, replication inhibitors, inhibitory antibodies,        antibodies directed against growth factors, bifunctional        molecules consisting of a growth factor and a cytotoxin,        bifunctional molecules consisting of an antibody and a        cytotoxin; cholesterol-lowering agents; vasodilating agents; and        agents which interfere with endogenous vascoactive mechanisms.    -   (b) Genetic materials include anti-sense DNA and RNA, DNA coding        for, anti-sense RNA, tRNA or rRNA to replace defective or        deficient endogenous molecules, angiogenic factors including        growth factors such as acidic and basic fibroblast growth        factors, vascular endothelial growth factor epidermal growth        factor, transforming growth factor alpha and beta,        platelet-derived endothelial growth factor, platelet-derived        growth factor, tumor necrosis factor alpha, hepatocyte growth        factor and insulin like growth factor, cell cycle inhibitors        including CD inhibitors, thymidine kinase (“TK”) and other        agents useful for interfering with cell proliferation the family        of bone morphogenic proteins (“BMP's”), BlVfiP-2, BMP-3, BMP-4,        BMP-5, BMP-6 (Vgr-1), BMP-7 (OP-1), BMP-8, BMP-9, BMP-10, BMP-1,        BMP-12, BMP-13, BMP-14, BMP-15, and BMP-16. Desirable BMP's are        any of BMP-2, BMP-3, BMP-4, BMP-5, BMP-6 and BMP-7. These        dimeric proteins can be provided as homodimers, heterodimers, or        combinations thereof, alone or together with other molecules.        Alternatively or, in addition, molecules capable of inducing an        upstream or downstream effect of a BMP can be provided. Such        molecules include any of the “hedgehog” proteins, or the DNA's        encoding them.    -   (c) Cells can be of human origin (autologous or allogeneic) or        from an animal source (xenogeneic) genetically engineered if        desired to deliver proteins of interest at the deployment site.        The cells may be provided in a delivery media. The delivery        media may be formulated as needed to maintain cell function and        viability.    -   (d) Suitable polymer materials as a coating or the base material        may include polycarboxylic acids, cellulosic polymers, including        cellulose acetate and cellulose nitrate, gelatin,        polyvinylpyrrolidone, cross-linked polyvinylpyrrolidone,        polyanhydrides including maleic anhydride polymers, polyamides,        polyvinyl alcohols, copolymers of vinyl monomers such as EVA,        polyvinyl ethers, polyvinyl aromatics, polyethylene oxides,        glycosaminoglycans, polysaccharides, polyesters including        polyethylene terephthalate, polyacrylamides, polyethers,        polyether sulfone, polycarbonate, polyalkylenes including        polypropylene, polyethylene and high molecular weight        polyethylene, halogenated polyalkylenes including        polytetrafluoroethylene, polyurethanes, polyorthoesters,        proteins, polypeptides, silicones, siloxane polymers, polylactic        acid, polyglycolic acid, polycaprolactone, polyhydroxybutyrate        valerate and blends and copolymers thereof, coatings from        polymer dispersions such as polyurethane dispersions (for        example, BAYHDROL® fibrin, collagen and derivatives thereof,        polysaccharides such as celluloses, starches, dextrans,        alginates and derivatives, hyaluronic acid, squalene emulsions.        Polyacrylic acid, available as HYDROPLUS® (Boston Scientific        Corporation, Natick, Mass.), and described in U.S. Pat. No.        5,091,205, the disclosure of which is hereby incorporated herein        by reference, is particularly desirable. Even more desirable is        a copolymer of polylactic acid and polycaprolactone.

While the invention has been described in terms of particular variationsand illustrative figures, those of ordinary skill in the art willrecognize that the invention is not limited to the variations or figuresdescribed. In addition, where methods and steps described above indicatecertain events occurring in certain order, those of ordinary skill inthe art will recognize that the ordering of certain steps may bemodified and that such modifications are in accordance with thevariations of the invention. Additionally, certain of the steps may beperformed concurrently in a parallel process when possible, as well asperformed sequentially as described above. Therefore, to the extentthere are variations of the invention, which are within the spirit ofthe disclosure or equivalent to the inventions found in the claims, itis the intent that this patent will cover those variations as well.

Finally, all publications and patent applications cited in thisspecification are herein incorporated by reference in their entirety asif each individual publication or patent application were specificallyand individually put forth herein.

1. An annular mesh that defines a lumen that surrounds a longitudinal axis of the mesh, the mesh being expandable radially from a compact diameter pre-deployment disposition to a radially expanded deployed disposition in which the mesh is capable of sustaining a radially outwardly directed resistive force even when flexing in response to externally applied forces that bend its longitudinal axis out of a straight line, the mesh being composed of stenting struts, the stenting struts being arranged in a plurality of zig-zag string around the circumference of the lumen, with occasional connector struts joining adjacent strings of stenting struts thereby to create a closed circumference unit cell between two such connector struts and two adjacent connected strings of stenting struts there being a plurality of such unit cells arranged in sequence around the circumference between said two adjacent string of stenting struts; and comprising a non-constant increment of strut length, within the portion of each of the two said zig-zag string of stenting struts bounding any one said unit cell said non-constant increment serving to displace along the longitudinal axis each unit cell bounded by said zig-zag string relative to the circumferentially next adjacent unit cell bounded by the same zig-zag string.
 2. The mesh according to claim 1, which is a stent for a lumen of the human or animal body.
 3. The mesh according to claim 1, wherein the length of the struts of each said zig-zag string portion is constant, except for one strut in each one of the two portions, which is shorter than the other struts of the said portion.
 4. The mesh according to claim 1, wherein the length of the struts of each zig-zag string portion is constant, except for two struts in each one of the two portions which are shorter than the other struts of the said portion.
 5. The mesh according to 1, wherein the increment of strut length between any two adjacent struts within each one of the two zig-zag string portions of said unit cell is finite and constant, except for an axial offsetting increment that lies between one and only one adjacent pair of struts of the said one string, which offsetting increment is different from said constant increment.
 6. The mesh according to claim 1, wherein the increment of strut length between any two adjacent struts within each one of the two zig-zag string portions of each unit cell is finite and constant, except for an axial offsetting increment that lies between two and only two adjacent pairs of struts of the portion, which offsetting increment is different from said constant increment, and not necessarily the same for both of the only two adjacent pairs of struts.
 7. The mesh according to claim 1, in which the mesh of struts is compatible with manufacture out of a seamless tube, by slitting the tube, through its wall thickness with a multiplicity of slits.
 8. The mesh according to claim 7, wherein most or all of the slits are straight.
 9. The mesh according to claim 8, wherein the straight slits are parallel to the long axis of the tube.
 10. The mesh according to claim 8, wherein the connector struts are also formable by slitting the tube.
 11. The mesh according to claim 10, wherein connector struts are straight.
 12. The mesh according to claim 11, wherein the straight connector struts are parallel to the long axis of the tube.
 13. The mesh according to claim 1, wherein the zig-zag strings face each other, in the compact pre-deployment disposition peak to peak yet, upon radial expansion to the deployed disposition, these facing peaks of the zig-zag strings move circumferentially away from the facing relationship and into a non-facing (peak-to-valley) relationship thereby enhancing the bending flexibility of the expanded mesh.
 14. The mesh according to claim 1, wherein each said string of stenting struts exhibits a plurality of circumferentially spaced apart three leg nodes, where a connector strut merges with the zig-zag string, the portion of the zig-zag string between two such nodes being oriented perpendicular to the longitudinal axis, whereby the nodes correspond to steps where the zig-zag strings advance axially along, the longitudinal axis.
 15. The mesh according to claim 14, wherein the closed circumference of each said unit cell including six said nodes.
 16. The mesh according to claim 1, wherein the unit cells axially one side of one of the zig-zag strings and the unit cells on the other axial side of the string are alike.
 17. The mesh according to claim 1, wherein the unit cells axially one side of one of the zig-zag strings are of one form, and those on the other side of that zig-zag string are of another form.
 18. An annular mesh that defines a lumen that surrounds a longitudinal axis of the mesh, the mesh being expandable radially from a compact diameter pre-deployment disposition to a radially expanded deployed disposition in which the mesh is capable of sustaining a radially outwardly directed resistive force even when flexing in response to externally applied forces that bend its longitudinal axis out of a straight line, the mesh being composed of stenting struts, the stenting struts being arranged in a plurality of zig-zag strings around the circumference of the lumen, with occasional connector struts joining adjacent strings of stenting struts thereby to create a closed circumference unit cell between two such connector struts and two adjacent connected strings of stenting struts there being a plurality of such unit cells arranged in sequence around the circumference between said two adjacent strings of stenting struts; and wherein each zig-zag string of stenting struts advances around the circumference in a pattern, which is neither continuously slanting to the longitudinal axis, as the bishop on a Chessboard, nor continuously perpendicular to the longitudinal axis, as the rook on a chessboard, but both perpendicular and across, as with the knight on a chessboard. 